gqrx, direct sampling configuration, shortwave/AM reception, etc

This is one of those articles that I write specifically to, you know, not forget it the next time.

I picked up a v3 RTL-SDR dongle from http://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/ a few weeks ago. It's a solidly looking aluminium can dongle with a much more useful RF connector. But, it still only tunes down by default to the same RTL-SDR limits as .. well, the rest.

So I go digging. I bought this thing because on Windows you're supposed to be able to tune down to a couple hundred KHz. I found out some useful stuff after a bit of google abuse:

• it's called "direct sampling mode";
• it's supported by the rtl_sdr driver/library that you normally use;
• you have to configure things up in a special way to get GQRX, etc to actually do the right thing;
• There's a specific thing called "Q-Branch" that you have to care about.

Ok, so the actual details.

Firstly, the NIC needs to go into direct sampling mode. So instead of being mixed with a VCO, you're bypassing all of that and just acting as a really fast ADC. Which, yes, will alias signals everywhere.

Secondly, you have to tell it to only give you "Q" samples, rather than both I and Q. If it's direct sampling, mixing doesn't come into it, so the hardware is just giving you ADC samples.

Then, you just tell GQRX, etc to ignore enforcing tuning limits (tick "No limits") and you're golden.

The specific string to add to the device string when you set up which rtl_tcp instance to talk to is:

direct_samp=2

(0 = no direct sampling; 1 = direct sampling on I, 2 = direct sampling on Q.)

Ok, so does it work? I'm still testing it. I will need to acquire some RF filters to try and filter out bands of HF frequencies to avoid aliasing as there is limited to on actual passband filtering going on here. But it did tune to AM radio and I picked up some data transmissions in 3MHz.

Baby steps!

gqrx on freebsd

I'm still tinkering with my own SDR code in C, but I decided I'd give gqrx a go. It sports a simple UI to visualise things as well as a simple control network protocol.

FreeBSD has packages for it, so it really was as simple as:

# pkg install gqrx

I already have the rtl-sdr bits installed, so that was pretty easy:

# rtl_tcp

.. to bring up the network service for it.

But running gqrx .. didn't work. It just stood there and did nothing when I ran it - no UI, no useful log output, nothing.

So out came truss. Step one in the "what is going on" showed no useful logging. Step 2 is "ok, what syscalls is it making." It turns out it was trying to connect to the IPv6 localhost address - but rtl_tcp defaults on FreeBSD to IPv4 localhost. gqrx doesn't handle connect() errors gracefully, so it was just stuck in an infinite connect() loop.

So I cleared the config, started gqrx again, and ensured this time it was set to '127.0.0.1' instead of 'localhost'. Voila! It worked!

Now - I need an SDR unit that can go down closer to DC so I can pick up shortwave. This RTL dongle I have doesn't tune below 27MHz.

RTL-SDR on FreeBSD, or "hey, cool, I live near an airport, I wonder if ADSB works.."

I bought one of those cheap RTL-SDR units a few months ago. There's no real kernel code required for it - all of the rtl-sdr code just uses the generic USB userland API which is shared between many operating systems.

So, getting it going was pretty easy:

# pkg install rtl-sdr

Then, using it to test ADSB is pretty easy:

# rtl_adsb -V -S 

.. this is verbose and listens to short packets.

Where I live (near San Jose Airport!) I receive a lot of ADSB transmissions. It's quite interesting.

Ok, so next - what about something more GUI like? Someone's already done it - https://github.com/antirez/dump1090 . There's already a package for it:

# pkg install dump1090
# dump1090 --net --aggressive

Then, point a webserver at http://localhost:8080/ and watch!